I would just vacume the sandbed a bit when you do a water change to stir things up a little. Like I said it doesn't usually spread on the sandbed - you might need a bit more flow in your tank. Coraline doesn't usually like really intense light although it will grow. I notice that it grows the best under VHO lights (at least in my tanks). Just make sure it isn't cyano - if it is vacume as much as you can out of there. Is it hard and crusty? - or slimey?